Transceiver systems generally comprise local oscillators for generating local oscillator signals at useful frequencies, e.g. of the order of a gigahertz, notably allowing frequency transpositions to be performed.
FIG. 1 represents an example of a common local oscillator DISo.
The local oscillator DISo comprises an oscillator module OSCo, in which an inductive element Lo and a capacitive element Co mounted in parallel form a conventional LC resonator known per se, generating an oscillating signal V1 at the terminals of the inductive element Lo.
The elements Lo and Co of such an oscillator module OSCo are usually selected to satisfy a compromise notably between phase noise, energy consumption and surface area occupied.
A sustaining amplifier SUST, supplied by a current source I, allows the oscillations of the signal V1 to be maintained by compensating resistive losses.
Such a setup offers good performance in terms of phase noise, energy consumption and surface area occupied by the circuit.
A frequency divider fDiv divides the frequency of the oscillating signal present on the inductive element Lo in order that the LC resonator can be configured for generating an oscillating signal V1 at a higher frequency. Indeed, this makes it possible to reduce the surface area occupied by the inductive element Lo and to reduce the coupling between the oscillator OSCo and the output signal of the divider fDiv or an amplified version of this signal, since the fundamental frequencies of the signals delivered by OSCo and fDiv are different.
The local oscillator signal LO is delivered at the output of the frequency divider fDiv, generally in the form of a square-wave voltage.
To produce such a signal the frequency divider fDiv must receive at its input a signal the amplitude of which is significantly above the thresholds of the transistors that it comprises.
However, high amplitude signals accentuate the “varactor” effect on the transistors of the input stage of fDiv, i.e. the variation in the capacitive component of their input impedance, which results in adding phase noise at the oscillator OSCo. More broadly this effect also occurs on all the other components connected to the LC resonator, such as the sustaining amplifier SUST and the amplitude controller MCTR. Thus, such local oscillators DISo are usually provided with a buffer amplifier ISO used both to isolate the LC resonator from the frequency divider fDiv inputs, and to generate an amplified signal V2 of sufficient amplitude for the input requirements of the frequency divider fDiv, from the oscillating signal V1.
Indeed, the capacitive value at the inputs of the buffer amplifier ISO is sufficiently small and sufficiently insensitive to the “varactor” effect not to interfere with the LC resonator.
On the other hand, this type of buffer amplifier ISO has the drawback of having a variable gain with many factors, such as temperature, the frequency of the signal at its input, or the vagaries of manufacture. Furthermore, this type of buffer amplifier ISO tends to degrade the duty cycle of the signal at its input (i.e., the ratio between the time at the high state and the period of the signal).